Bell-Ringer In your group, use the paper, marker, and anything else within the room to model a divergent boundary.
Early Evidence of Continental Drift 1620 –Sir Francis Bacon noticed that continent coasts fit like puzzle pieces --geological formations/ fossils match up across Atlantic ocean 1912, Wegner’s hypothesis of continental drift and “supercontinent” called Pangaea (didn’t explain HOW continents moved) ’s: Theory of Plate Tectonics
Pangaea
Sonar After World War II, scientists used sonar to survey large areas of the sea floor. Using sonar, they discovered……. Sends a signal (sound) toward the ocean floor and analyzes the return signal (echo) that bounces off the sea floor or another submarine structure
Plate boundary 1: Divergent or Mid-Ocean Ridge Continuous chain of submarine volcanic mountains that circles the globe Mid-ocean ridge system is largest geological feature on earth
Some of the earth’s major mid-ocean ridges
New sea floor is created at Mid Ocean Ridges Rifts are cracks in the earth’s crust Created at MOR, where pieces of oceanic crust separate When a rift occurs, pressure from the mantle underneath the rift is released Pressure release causes magma to push upward through the rift Magma cools & solidifies new sea floor!
Sea Floor Spreading Process in which the sea floor moves away from a MOR as magma pushes it outward, and new sea floor is created
Magnetism Earth’s magnetic field reverses direction about every 700,000 years Cause related to movements of magnetic particles in earth’s molten (liquified) outer core Many rocks contain tiny magnetic particles Particles can move when rock is molten Rocks are molten at MORs Particles are frozen in place when rock cools
Magnetism (continued) Even when earth’s magnetic field reverses direction, the rocks stay in place Results in magnetic “bands” in the sea floor, running parallel to MOR Bands alternate between “normal” and reverse magnetization Allows scientists to determine orientation of earth’s magnetic field when the rocks cooled Further supports sea floor spreading idea
Plate Boundary 2: Convergent or Trenches If new sea floor is created, old sea floor has to be destroyed somewhere else Old lithosphere destroyed at trenches Two plates collide, one plate dips below other into mantle (subduction) Plate breaks up & melts in the molten mantle Breaking up causes earthquakes Melting causes volcanoes
Continued… When a continental plate collides w/ oceanic plate, the oceanic plate ALWAYS subducts Why? Continental crust is less dense than oceanic crust and sits up higher Causes the development of continental volcanoes ANDES
Cont’d… When two oceanic plates collide, one plate subducts -casues earthquakes & volcanoes When two continental plates collide (only occurs on occasion)… because both have a low density, neither tends to subduct Instead, plates get “welded together” or cause buckling and fold like an accordion Forms mountain ranges (Example: Himalayas formed when India collided with Asia)
Plate boundary 3: Shear boundary or transform boundary Plates slide past each other Neither creates nor destroys lithosphere Builds friction that eventually “gives,” causing an earthquake Ex: San Andreas Fault
Hydrothermal Vents Seawater seeps into cracks in floor near the rift at mid-ocean ridges heated by underlying mantle Forces back up through crust at 50-68°F, sometimes 660°F
Hydrothermal Vents Seawater seeps into cracks in floor near trenches and the rift at mid-ocean ridges heated by underlying mantle Forces back up through crust at 50-68°F, sometimes 660°F
Hydrothermal vents cont’d… Water that’s seeped into cracks mixes with minerals (sulfides) This hot water then emerges from the vent and solidifies when it meets the cold ocean water surrounding the vent The minerals solidify too, forming mineral deposits, which form structures near the vent
Black smokers A type of mineral deposit found at hydrothermal vents Chimney-like structures “smoke”= dense cloud of mineral particles Largest black smoker = 200 ft above sea floor